The northern cities of Calama and Arica lost electrical power and panicked residents ran into the streets.
Fernandez said the quake stretched across nearly 2,000km, from Chile’s northern border with Peru to the capital.
The Pacific Tsunami Warning Centre in Hawaii said that a tsunami warning issued earlier had effectively expired with no major or damaging waves arriving within two hours of the quake.
However, “danger to boats and coastal structures can continue for several hours due to rapid currents,” it said.
Martinez said a strong 5.7 magnitude aftershock hit the region, just two hours after the first earthquake.
USGS said the aftershock was centered 58km northwest of the town of Antofagasta, about 200km from Calama, and struck at 1744 GMT.
Another aftershock at 4.5 on the Richter scale hit the western Argentine city of San Juan, with no injuries immediately reported.
Residents of San Juan poured into the streets, the national earthquake office said.
Earlier this year, a quake measuring 8.0 hit southern Peru, killing about 540 people and leaving another 200,000 homeless.